Preparation of papers for special issues of IEEE development of microresonator detectors for 163 Ho endpoint measurement in Milano

The determination of the neutrino mass is still an open issue in particle physics. The calorimetric measurement of the energy released in a nuclear beta decay allows measuring the whole energy, except the fraction carried away by the neutrino: due to the energy conservation, a finite neutrino mass m(nu) causes the energy spectrum to be truncated at Q-m(nu), where Q is the transition energy of the decay. The electron capture of Ho-163 (Q similar to 2.5 keV) is an ideal decay, due to the high fraction of events close to the endpoint (i.e., the maximum energy of the relaxation energy spectrum). In order to achieve enough statistics, a large number of detectors (similar to 10(4)) are required. Superconducting microwave microresonators are detectors suitable for large-scale multiplexed frequency-domain readout, with theoretical energy and time resolution on the order of electronvolts and microseconds, respectively. Our aim is to develop arrays of microresonator detectors applicable to the calorimetric measurement of the energy spectrum of Ho-163. Currently, a study aimed at the selection of the best design and material for the detectors is in progress. In order to obtain low-T-c detectors, with T-c ranging between similar to 0.5 and 2 K, different Ti/TiN (titanium nitride) multilayer films were produced. The reduced T-c was obtained by superposing thin layers of stoichiometric TiN to pure Ti layers, and the T-c was tuned by varying the ratio between the thickness of the layers. In this contribution, a comparison between the measurements (critical temperature, gap parameter, and X-ray energy spectra) made with stoichiometric and substoichiometric TiN and Ti/TiN multilayer film microresonators is presented.